The invention relates to a method of manufacturing a magnetic, plastic-bonded molded body which during its manufacture can be adjusted as regards its magnetic properties. A powdered magnetic material is provided which is mixed with a plastic binder to form a mass which is then subjected to a shaping process. This shaping process may be in particular extrusion or injection molding.
During the manufacture of molded bodies from mixtures of plastic and magnetic materials, for example, plastic and ceramic powders of permanent magnetic material, two criteria are of particular importance for the user of such molded bodies:
(a) the magnetic properties, and
(b) the physical (mechanical) dimensions and the size tolerances.
In a manufacturing process, the values of these criteria are selected in (a) by controlling the quantity of the component of magnetic material in the starting mass, and in (b) by accurately following the process specification during the shaping of the plastic and by accurately forming the shaping tools.
When molded bodies having different magnetic properties but having the same physical dimensions are to be manufactured, the problem arises that the tools have to be changed since, when the same plastic components are used, a change in the quantity of magnetic material in the mixture of the components causes a different shrinkage behavior of the mass during and after shaping.
Masses having different components of magnetic material thus result in products having different dimensions, when they are to be shaped by means of the same tool. Variations in the shaping conditions, such as plasticizing the mass, temperature of the mass, temperature of the tool, pressure during shaping, and pressure during the compaction step after shaping, cannot compensate for these differences to such an extent that the size of all products will lie within a uniform narrow tolerance range.
It is known that desired decreased magnetic strength of molded bodies with a given filling of magnetic material can be obtained by using fields with decreased field strengths during the magnetization of the molded bodies. However, this method has the disadvantage that the magnetization takes place in the range of the steep slope of the magnetization curve (1st quadrant of the hysteresis curve) where small fluctuations in the magnetization energy result in large tolerances in the desired magnet strength.
It is an object of the invention to solve this problem and to provide a method by means of which molded bodies having the same physical dimensions but different magnetic properties can be manufactured by means of the same tool.